In the construction of a radar detector of the type which is adapted to be carried on the dashboard of a motor vehicle for detecting radar signals being transmitted from a police radar traffic control unit, it is common for most manufacturers to use a horn type antenna. A typical horn antenna includes a tapering wall portion in the form of a frusto-pyramid. The wall portion defines a converging cavity which extends to a rectangular waveguide cavity in which a pair of diodes are mounted for detecting and modulating a microwave signal.
In such radar detectors, it is desirable to minimize the size of the housing which has an aperture aligned with the horn cavity to minimize the depth of the detector. Thus the length of the horn antenna is substantially reduced from the optimum design for the horn antenna with the result that a sacrifice is made in the sensitivity or gain of the antenna, and weak signals are not detected. For example, the radar detectors marketed under the names "FUZZBUSTER", "WHISTLER", "BEARFINDER" and "RADAR RANGER" each include a horn antenna which has been substantially shortened from an optimum design in order to fit within a relatively compact housing which may be mounted on the dashboard of a motor vehicle.
The lost gain or sensitivity in the relatively short horn antenna is due to phase errors at the entrance or mouth of the antenna and in the use of improper angles for the tapering walls of the antenna in addition to matching errors at the junction of the tapered cavity with the rectangular wave-guide cavity of the antenna. These errors result in higher sidelobe levels and/or less energy available for detection by the detector diode.